Free way to create captions, but more time consuming:
Create a Camtasia video with your audio file and a placeholder text box, then upload to YouTube, which will automatically caption your video. You can edit it before downloading the file in .vtt or .srt format.
Last week I found this software, Descript. They have a free account, but it is pretty restrictive. So, I purchased the pro version. If you have the script, it generates a captioning file in less than a minute! If not, it generates a caption file in 1-2 minutes. You can then edit the file in the program. Highly recommended!